FIELD OF THE INVENTION
The invention relates to a circuit for producing a reset signal, having a voltage divider at which a voltage proportional to a supply voltage can be tapped off, and a low-pass filter configuration.
Such a circuit is necessary in any electronic device which has components that are able to process digital signals. When the device switches from a turned-off state to a turned-on state, undefined and/or impermissible states which are to be avoided may occur in those components. There is therefore a need for a circuit which, during a rise of the supply voltage to its rated value, places those components in a predefined state from which operation can be started once the rated value of the supply voltage has been reached.
Typically, the components in question have a reset terminal which places the component concerned in a predefined state when a specific level is applied to that terminal. If no such level is applied at a given time, the component is in a normal operating state. Reset circuits are known having an output which remains at the specific level until the supply voltage has reached its rated value. In that regard, reference is made, by way of example, to a book entitled "Halbleiter-Schaltungstechnik [Semiconductor Circuitry] by Tietze and Schenk, 7th edition, 1985, Chapter 21.1.1 "Reset-Logik" [Reset Logic], pages 637 to 639. Another circuit configuration for producing a reset signal is disclosed, for example, in German Patent DE 44 43 606 C1.
In practice, such reset circuits are obtainable as separate components. That is disadvantageous, however, because such an additional component takes up a corresponding amount of space on a printed circuit board. That is an obstacle to the aim of smaller and smaller module sizes and higher integration densities. Furthermore, reset circuits should also output a reset signal whenever the supply voltage drops below a specific threshold. That function is known as undervoltage detection.
U.S. Pat. No. 4,727,270 discloses a pulse sensor which is insusceptible to noise. The sensor has a series circuit including a first Schmitt trigger, a low-pass filter and a second Schmitt trigger. An output signal from the second Schmitt trigger is fed back to an input signal for the first Schmitt trigger. The circuit disclosed is used for receiving and initial processing of a sensor signal.